Multiemployer plan (Details 2) - Multiemployer plans, pension [Member] - Operating Engineers Local 800 and WY Contractors Association Pension Plan for WY [Member] - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2015	Jun. 30, 2016
Multiemployer Plans [Line Items]
Additional multiemployer plan withdrawal obligation	$ 2.4
Multiemployer plans, withdrawal obligation		$ 16.4

The amount of loss pertaining to the specified contingency that was charged against earnings in the period, including the effects of revisions in previously reported estimates.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Name:	us-gaap_LossContingencyLossInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration